When a magnetic disk is rotated at a high speed, a lubricant of a lubrication film formed on a surface of the magnetic disk tends to be splashed easily by a shearing force caused by air and a centrifugal force thereof.
As prior art for increasing molecular weights of lubricants, methods for removing low molecular weight components and impurities by solvent extraction are disclosed in JP No.267707, JP-A Nos. 4-31005, 5-20673, 5-234066, 5-109053, 6-215347, 7-141644, 9-291296, 9-157674, and 10-53781.
Lubricants that have been polymerized to have a high molecular weight by the solvent extraction method described in JP No.2677074, however, have such a problem that a substitution ratio of its end-group""s function group drops depending on its type of lubricants, and as a result, a quantity of splash of its lubricants cannot be reduced.
Further, in order to enable a molecular weight control of a substantial amount of lubricants for industrial use by means of a splitter liquid chromatography, gel permeation chromatography, or evaporation method, provision of large-scaled facilities is required, which is economically disadvantageous.
The present invention relates to lubricants to be coated on a magnetic disk, such a magnetic disk, and a magnetic disk apparatus using the same.
For magnetic disks now available on the market, perfluoropolyether lubricants having a numeric average molecular weight from 2000 to 6000 are widely used. For example, Fomblin Z-DOL (trademark), Fomblin AM2001 (AUSIMONT S.p.A) the like are known. These perfluoropolyether lubricants available on the market have a molecular weight distribution from about several hundreds to 15000. Components of the lubricants easily spin out by a shearing force caused by high speed rotation or by a temperature rise in the apparatus are those with a low molecular weight smaller than about 2000. Therefore, in order to minimize splashing of lubricants, it is necessary to use a lubricant from which a low molecular weight component that is easily to spin out is removed in advance prior to its use for forming a lubricant film on the surface of a magnetic disk.
Further, in views of reliability in slidableness property of the magnetic disk, it is necessary to remove a certain amount of high molecular weight composition on the high molecular weight side as well. With a decrease in floating level, a magnetic head slider tends to make a contact slide during a seek time, thereby easily allowing lubricants present on a disk surface to be collected at a slider portion of the slider. If a viscosity of the lubricants collected (of a high molecular weight) becomes higher, its sticktion becomes stronger. Therefore, both components of one having a low molecular weight that is easily splashed and of the other one having a high molecular weight that results in a strong stickiness must be removed.
Further, even if its average molecular weight and molecular weight distribution of the lubricant are controlled as described above, a quantity of splash cannot be reduced because a substitution ratio of an end-group""s functional group that secures an adsorption force of the lubricant is decreased by the solvent extraction method and the like. Therefore, while controlling the average molecular weight and molecular weight distribution such that the splash of the lubricant is minimized, it is also required at the same time to prevent a drop in the substitution ratio of the end function group.
An object of the invention is to provide for a lubricant that has a high substitution ratio for its end function group, a magnetic disk that has a minimum quantity of splash of the lubricant and has an improved reliability of slidableness, and also a magnetic disk apparatus using same.
A lubricant according to the invention is a perfluoropolyether expressed by the following formula (1), in which a component of molecular weights less than 1000 is 10 weight % or less, a component of molecular weights above 7000 is 15 weight % or less, and their molecular weights are controlled such that a ratio between a weight average molecular weight and a numeric average molecular weight becomes 1.5 or less, and a substitution ratio of its terminal functional group becomes more than 90%.
R1xe2x80x94CF2Oxe2x80x94[xe2x80x94(CF2CF2O)mxe2x80x94(xe2x80x94CF2O)nxe2x80x94]xe2x80x94CF2xe2x80x94R2xe2x80x83xe2x80x83(1)
where, R1 and R2 are univalent organic groups, and m and n are positive integers.
Structural formula (1) may also be perfluoropolyether having structural formulas of (2) or (3) below.
HOCH2xe2x80x94CF2Oxe2x80x94[xe2x80x94CF2CF2Oxe2x80x94]xe2x80x94mxe2x80x94[xe2x80x94CF2Oxe2x80x94]xe2x80x94nxe2x80x94CF2xe2x80x94CH2OHxe2x80x83xe2x80x83(2)

where m and n are positive integers.
Further, another lubricant according to the invention is a perfluoropolyether expressed by the following formula (2), in which its molecular weight is controlled such that a numeric average molecular weight is not less than 5500, a composition of a molecular weight of smaller than 3000 is 15 weight % or less, a ratio between a weight average molecular weight and a numeric average molecular weight is less than 1.5, and a substitution ratio of its end function group is not less than 90%.
HOCH2xe2x80x94CF2Oxe2x80x94[xe2x80x94CF2CF2Oxe2x80x94]xe2x80x94mxe2x80x94[xe2x80x94CF2Oxe2x80x94]xe2x80x94nxe2x80x94CF2xe2x80x94CH2OHxe2x80x83xe2x80x83(2)
where, m and n are positive integers.
A Method of Controlling Molecular Weights of Lubricants:
According to the invention, an average molecular weight and molecular weight distribution of a lubricant are controlled using a ultra filtration method. According to this ultra filtration method, without need of a large-scaled facilities, a lubricant solution can be divided into a prescribed molecular weight simply by filtering and circulating through ultrafilter at a constant pressure and flow rate.
More specifically, a solution of perfluoropolyether dissolved in a fluorine-containing solvent is circulated through a ultrafilter made of polyethersulfone film having uniform pores under a constant pressure so that the lubricant is divided into a group of molecular sizes that passes through the pores and into another group of molecular sizes that does not pass through the pores.
Various films having a various pore size are available for use as the ultrafilter, therefore, by combining some of them, it becomes possible easily to control and obtain an appropriate average molecular weight and molecular weight distribution as desired. By the way, the ultrafilter film made of polyethersulfone used in the invention is a hydrophilic film. Generally speaking, it is substantially difficult for a hydrophilic film to permeate a hydrophobic substance such as a perfluoropolyether lubricant that contains fluorine element at a high concentration, fluorine containing solvents and the like.
However, it is enabled according to the invention to allow for the hydrophilic ultrafilter to permeate a hydrophobic substance. Firstly, a pure water is permeated through the ultrafilter made of polyethersulfone. A main object of this operation is to allow for water molecules to adsorb on the surface of the ultrafilter.
Then, a mixture of solvents of a fluorine-containing solvent and an alcohol solvent is circulated through the surface of the ultrafilter under application of a pressure. As its fluorine-containing solvent, a hydrofluoroether group solvent such as HFE-7100, HFE-7200 etc. that are compatible with alcohol solvents, or hydrofluorocarbon group solvents such as Bartlel XF etc. are used. As the alcohol group solvent, preferably, methylalcohol, ethylalcohol, isopropylalcohol and the like that can mix easily with the above-mentioned hydrofluoroether group solvent, hydrofluorocarbon group solvent or the like are used.
Through these operations according to the invention, water molecules present in the surface of the ultrafilter are substituted by alcohol, thereby modifying the surface of the ultrafilter film to have such a property, although it is inherently hydrophilic, that has affinity to a hydrophobic flurorine-containing solvent (such as hydrofluoroether group solvents) as well. Namely, the hydrofluoroether group solvents having no affinity to water molecules can be modified to be able to pass through the filter by substitution with alcohol that is hydrophilic and compatible with hydrofluoroether group solvents.
By way of example, it may be contemplated to circulate this mixture of solvents from the first stage, however, in order to ensure for alcohol molecules to be uniformly adsorbed, and to remove glyceline coated for prevention of drying of the polyethersulfone film, or to remove contaminations such as alkali ion metals and the like, flow of pure water prior to the mixture of solvents is preferred.
Types of Magnetic Disks:
According to the invention, lubricants are coated on the surfaces of two types of magnetic disks (type (a) and type (b)) to form a lubrication film thereon.
A magnetic disk 29 of type (a) has a structure, which is comprised of underlayer 24 made of Cr alloy formed on a glass substrate 23, magnetic layer 26 made of such as CoCrTaPt etc. formed thereon, protection film 27 having carbon mainly, and lubrication film 28 made of perfluoropolyether formed thereon as an outermost layer as shown in FIG. 4. In the magnetic disk of type (a), which is to be mounted on a load/unload type magnetic disk apparatus in which its magnetic head slider is present on its disk plane only during disk rotation, and is always evacuated out of the disk plane during disk stoppage, there is no need to take into account sticktion (sticking) between its magnetic disk and its magnetic head slider. Therefore, in the magnetic disk of type (a), there is no need to form bumps for prevention of sticktion on the disk surface, therefore, its disk surface is flat and smooth.
The magnetic disk of type (b) has a structure that is provided by forming: a NiP film and a Cr film sequentially on the surface of an aluminum alloy substrate as an underlayer 16; a magnetic layer 17 made of CoCrTaPt and the like thereon; a protection film 18 made mainly of carbon; and a lubrication film 19 made of perfluoropolyether as an outermost layer (see FIG. 2). Because this magnetic disk of type (b) is to be installed in a contact start stop type magnetic disk apparatus in which its magnetic head slider floats on the surface of the disk during disk rotation and stands by in contact with the surface of the disk during disk stoppage, it is required to take into account sticktion (sticking) between the disk and the magnetic head slider when starting the disk. Therefore, a plurality of ring-like bumps with a regularity are formed in a band with less than 5 mm width in an internal periphery portion with a radius of more than 10 mm and less than 25 mm, which will be referred to as a laser zone texture hereinafter. In this laser zone texture area, there occurs no strong sticktion. The magnetic head slider is ensured always to return to this laser zone texture area to stand by at the time when the disk is at rest.
The above two kinds of magnetic disks of type (a) and (b) are installed in two kinds of magnetic disk apparatuses of type (A) and (B), respectively.
In a magnetic disk apparatus of type (A), its magnetic head slider is of a load/unload type that evacuates the magnetic head slider out of the disk plane in the periphery of the disk at the time when the disk stops, and there is provided a ramp that supports a gimbal connected to the magnetic head slider at the time of evacuation thereof.
In a magnetic disk apparatus of type (B), its magnetic head slider is of a contact-start-stop type, in which the magnetic head slider floats on the surface of the disk during operation of the disk and stands by at rest in contact with the disk during disk stoppage.